When exchanging messages between computing devices such as mobile devices or desktop computers, often the messages being exchanged are cryptographically processed prior to being transmitted, and complementary processing applied upon receipt. For example, it is well known to encrypt a message using a symmetric key. The cryptographic processing being used often needs to have consideration for the cryptographic protocols/algorithms supported not only by the sender, but also any recipients. In some cases, numerous cryptographic protocols are available to be used and the sender typically chooses the cryptographically “strongest” protocol that would be supported by both sender and receiver. An example of such a scheme is the Secure/Multipurpose Internet Mail Extension (S/MIME) scheme, which is a well known standard for public key encryption and signing of email encapsulated in MIME.
In schemes such as S/MIME, when sending a message to a recipient that the sender has not communicated with before, a default algorithm required to be supported by all recipients is typically used, e.g. RC2 or 3DES. Some mail clients that use S/MIME send their S/MIME capabilities with their messages, which includes an indication of which algorithms or protocols are supported by that client. This enables the recipient to use this information to send further messages accordingly.